XFilesharingPro embedded URLs

Apparently, XFilesharingPro hosters allow embedding their files, resulting in URLs that are not detected by the current XFilesharingPro hook. These commits should fix that for everything that's handled via XFilesharingPro. I don't know about hoster plugins that inherit from the XFilesharingPro hoster plugin...
I think it's better to append this to `FILE_URL_REPLACEMENTS`, then the next line will execute it.